What Are Good Sources of Protein? – Speed of Digestion Part 1

In the last article, What are good sources of protein – Digestibility, I examined some basics of protein digestibility and presented data on the gross digestibility of varying proteins. Summing up, animal source proteins such as meat, milk and eggs show extremely high (90%+) digestion while vegetable source proteins show much lower values.

However, the efficiency of digestion alone is not the only factor which goes into answering the question What are good sources of protein?

Recently (and by that I mean the late 90’s or so), an interest in the speed of digestion and how that impacts on various aspects of human physiology has occurred. It’s turning out that proteins can digest at fairly different rates and this turns out to affect various physiological processes; the main two are protein synthesis and protein breakdown. As with the last article, I’m going to talk about these terms in brief before moving onto the main thrust of today’s article.

Because I have a lot of information to cover, I’m going to break the topic down into two parts. In Part 1 today, I need to cover a bit more background physiology and talk about the original study that kicked off the entire interest in speed of digestion. In Part 2 (tomorrow), I’ll finish up with some other recent developments. As always, all of this information can be found in a more detailed and expanded fashion in The Protein Book.

Protein Turnover: The Combination of Protein Synthesis and Protein Breakdown

Early ideas about the body held that the different tissues such as fat cells and skeletal muscle (see What Does Body Composition Mean for a little more detail) were essentially static and unchanging. This turns out to be incorrect. At any given time in the body, pretty much all of the cells in your body are undergoing a constant process of breakdown (where larger structures are broken down into smaller) and synthesis (where smaller structures are combined to make larger).

So as you sit here reading this, your fat cells are both breaking down and re-synthesizing the triglyceride (fat) stored in them. Bone is undergoing the same constant processes as well. Of course, the same holds for protein tissues.

Right this moment, your liver is breaking down and remaking various proteins, and your skeletal muscle is in a constant source of breakdown and re-synthesis. While this is actually energetically very costly, and seems wasteful, it turns out to give the human body an incredible adaptability and ability to deal with stress.

The combination of breakdown and re-synthesis is referred to, generally, as turnover. In the context of protein based tissues, this is referred to as protein turnover.

I should note that different tissues in the body break down at drastically different rates. So while liver proteins may break down and be completely re-synthesized in a number of hours, skeletal muscle is turning over more slowly. Tissues such as organs, tendons and ligaments turn over much more slowly. As you’ll see, this actually has some implications for what I’m going to talk about in just a moment.

What happens overall to a given tissue (e.g. whether it grows, shrinks or stays the same) depends on the relative rate of synthesis and breakdown. Simply:

If synthesis is greater than breakdown, there will be an increase in the amount of that tissue.

If breakdown is greater than synthesis, there will be a decrease in the amount of that tissue.

If breakdown equals synthesis, there will be no change in the amount of that tissue.

This also means that, fundamentally, we have two different ways to have an impact on the amount of a given tissue. Let’s say for example that someone wants to increase the amount of muscle that they have. They can either try to increase protein synthesis, decrease protein breakdown, or cause both to occur.

This is an important distinction because various things (such as nutrients, training, drugs) can differentially affect each process. As you’ll see in just a second, speed of digestion is one of those things and the rate of digestion of a given protein can affect protein synthesis vs. breakdown differently.

The Now Infamous Boirie Study

Back in 1997, a research group in France published the first paper on the topic of slow and fast proteins. Titled, “Slow and fast dietary proteins differently modulate postprandial protein accretion.”, this is the paper that kicked off the entire field of fast and slow proteins.

In that paper, subjects were fed either casein or whey, the two proteins found in milk (see Milk: The New Sports Drink? A Review – Research Review for more information), and blood amino acid level along with whole body protein synthesis and breakdown were measured. I’d note that both proteins were given after a morning fast with no other nutrients (carbohydrates or fat) provided. This is important because the results of this study don’t necessarily hold when other nutrients are being consumed, or someone is consuming a given protein later in the day (when other meals are still digesting).

The researchers found the following: whey spiked blood amino acid levels faster than casein, but blood amino acid levels dropped more quickly as well. Casein, in contrast, took much longer to digest, actually providing amino acids for around 8 hours to the body (you might consider that data point the next time you hear that you have to eat every three hours or your muscles will fall off, a topic I cover in Meal Frequency and Energy Balance).

I actually want to clarify that a bit since there has been a lot of confusion over what the study actually found. Both casein and whey hit the bloodstream at about the same time (about an hour in), that is, whey didn’t actually get into the system faster. However, whey spiked blood amino acid levels higher at that one hour point. Figure 1 (taken from The Protein Book) shows this.

Amino acid profile for casein vs. whey

Note that both proteins enter the bloodstream at about the same time, around the one hour mark. Whey simply spikes blood amino acids faster (before falling back to baseline levels around hour 4). Casein, in contrast raises amino acid levels to a much lower level but they are sustained for hours (in the graphic, at the 7 hour mark, blood amino acid levels were still above where they started).

So it’s not that whey gets into the system faster, it just spikes blood amino acid levels higher at the same time point (about an hour after consumption).

Now, the next bit of this study was an examination of the effects of these proteins on protein synthesis and breakdown. Basically, it was found that whey raised protein synthesis with no effect on protein breakdown while casein decreased protein breakdown without affecting protein synthesis.

Hence, whey become known as an ‘anabolic’ protein (anabolic just means making bigger things out of smaller things) and casein was an ‘anti-catabolic’ protein (catabolic means making smaller things out of bigger things, and anti-catabolic means that casein prevents that).

I’d also note that more of the whey was burned for energy (oxidized) compared to the casein.

This, of course, got taken wildly out of context to sell protein powders. However, note above I said that the research was looking at whole-body protein synthesis and breakdown. It wasn’t examining skeletal muscle per se. It’s just as logical to conclude that the whey stimulated liver protein synthesis as skeletal muscle but, of course, supplement companies don’t ever talk about that.

And with that I’m going to wrap up Part 1. In Part 2 (tune in tomorrow), I’ll talk about more recent research and some implications of speed of digestion for answering the question What are good sources of protein?